1. Field of the Invention
This invention relates to an improved motor speed control system and more particularly to a motor speed control system for an electronically commutated brushless D.C. motor having bi-directional multi-speed control and a resolver which provides both phase-locked servo feedback and motor commutation.
2. Description of the Prior Art
There are many applications where precise control of electric motor speed is necessary.
In an orbiting spacecraft, for example, where solar arrays are utilized to convert solar energy into electrical energy it is necessary to rotate the array during the spacecraft's orbit so as to obtain optimum exposure to solar energy. For a spacecraft in a geo-stationary orbit, the solar array shaft is rotated such that it completes one revolution in each 24 hour period. Therefore, it becomes necessary to provide a motor speed control system which is able to control angular shaft position with precision at extremely slow motor speeds.
One system for controlling motor speed is described in U.S. Pat. No. 3,828,234 issued on Aug. 6, 1974, to RCA Corporation, based on the invention of Edwin Goldberg entitled "Motor Speed Control System." The system described therein achieves control by using an error signal voltage which is a function of the phase difference between the output voltages of a resolver and a suitable phase shifting circuit. The error signal voltage is then amplified and compensated by suitable motor electronics. The motor electronics output signal is then used to energize the motor and thereby turn the motor shaft until the magnitude of the error signal voltage is either nulled or decreased to some predetermined value.
According to the present invention, an improved bi-directional multi-speed motor speed control system is described wherein a resolver provides both phase locked servo feedback and motor commutation for an electronically commutated brushless D.C. motor.